Measure the inside of the cavity in millimeters for best accuracy.
Formulas
Box: L×W×H
Cylinder: π×r²×H
Sphere: 4/3×π×r³; dome is half that
Pyramid: L×W×H÷3; cone: π×r²×H÷3
1,000 mm³ = 1 ml. For odd shapes (skulls, geodes, letters) fill the mold with water, pour into a measuring cup, then dry the mold completely — any moisture clouds epoxy.